Speciation of exchangeable and crystalline Fe oxide bound Zn, Cu, Fe and Mn ions from calcareous soils during sequential fractionation

The use of 1 M neutral NH₄OAc has been criticized for extraction of exchangeable ions in calcareous soils as it dissolves free CaCO₃ from soils. As a result the metal ions (i) occluded on CaCO₃ and (ii) by their additional exchange from solid phases owing to an increased concentration of free Ca²⁺ ions in solution, are released during extraction. In order to develop a suitable substitute of 1 M neutral NH4OAc, it was hypothesized that the extraction of soils with Pb(NO₃)₂ at a concentration <0.05 M for a finite time, prior to their extraction with frequently used 0.05 M Pb(NO₃)₂ (pH 6.0) for extraction of specifically adsorbed cations, should be able to extract the exchangeable metals from soils. Of the various concentrations (0.0025, 0.005, 0.01, 0.02 and 0.04 M) of Pb(NO₃)₂ and shaking intervals (15, 30, 60, 90 and 120 min) tested, the extraction of soils with 0.005 M Pb(NO₃)₂ (pH 6.8) for 15 min was adjudged to be satisfactory for estimation of exchangeable Zn, Cu, Fe and Mn. The efficacy in the extractability by 0.005 M Pb(NO₃)₂ (pH 6.8) was compared with that of 1M neutral NH₄OAc. Compared to 0.005 M Pb(NO₃)₂ (pH 6.8), a significant increase in the extraction of exchangeable Zn (16.2%), Cu (70.9%), Fe (62.4%) and Mn (76.1%) with 1 M neutral NH₄OAc was observed. However in subsequent steps, the extraction of Zn (47.5%), Cu (49.1%) and Mn (22.9%) associated with free CaCO₃ of soils decreased significantly when exchangeable metals were extracted with 1 M neutral NH₄OAc. Thus, a considerable part of these metals associated with free CaCO ₃ in soils may be considered in exchangeable form estimated with 1 M neutral NH₄OAc. These findings were further substantiated by the detection of a significantly higher mean concentration of 130 me HCO₃⁻ and 32.2 me Ca² + + Mg² + ions L⁻ ¹ in 1M neutral NH₄OAc extracts compared to 1.66 me HCO₃⁻ and 16.2 me Ca² + + Mg² + ions L⁻¹ in 0.005 M Pb(NO₃)₂ (pH 6.8) extracts. The use of 0.1 M ascorbic acid as a reducing agent was tested with 0.25M NH₂OH.HCl + 0.25M HCl to extract Zn, Cu, Fe and Mn associated with crystalline Fe oxides.